Osteoporosis affects 10 million Americans, and associated fragility fractures cause significant morbidity and mortality. While a great deal is known about the molecular pathways controlling osteoblast and osteoclast function, relatively little is known about the most abundant cell type in bone, the osteocyte. Osteocytes produce sclerostin, a potent inhibitor of bone formation by osteoblasts and an osteoporosis drug target. Notably, the only existing osteoporosis therapy that boosts bone formation, parathyroid hormone (PTH) 1-34 (teriparatide), works in part by reducing sclerostin production by osteocytes. The focus of this grant proposal is to elucidate the intracellular mechanisms controlling osteocyte biology, with an emphasis on sclerostin gene regulation and its control by PTH. Class IIa HDACs (Hdac5 and Hdac4) have been identified as important regulators of osteocytic sclerostin production and osteocyte differentiation in general. Aim 1 will fully characterize the effects of class IIa HDAC deficiency on osteocyte biology in vivo using multiple independent and complementary approaches. Aim 2 will interrogate the role of class IIa HDACs in PTH-mediated sclerostin suppression in vivo. Importantly, though some information is known about how class IIa HDACs might control sclerostin expression, much remains unknown. Therefore, Aim 3 will determine novel class IIa HDAC binding proteins and substrates in osteocytes using cutting-edge proteomic approaches. The candidate Dr. Wein is a physician/scientist dedicated to a career in basic investigation in skeletal biology. His education (combined MD/PhD degrees) provides him rigorous training in basic science and human physiology. An endocrinologist subspecializing in metabolic bone diseases, considerable and complementary overlap exists between his clinical and research interests. Beyond the research proposed within, Dr. Wein's career development plan will allow him to maximize the resources within the MGH Endocrine Unit, Harvard Medical School, and the Broad Institute to achieve his career goals. He has identified a mentor, Dr. Henry Kronenberg, who is a worldwide leader in skeletal biology and PTH actions. Dr. Wein and Kronenberg have a plan for frequent meetings to discuss data and career development. Dr. Kronenberg has clearly identified aspects of the research proposed by Dr. Wein which will form the basis of his independent career. Collaborators within the MGH Endocrine Unit and at the Broad institute have been identified, and an advisory committee has been formed to evaluate progress and plan future directions. Dr. Wein will frequently present his data and attend seminars and journal clubs in the MGH Endocrine Division and at the Broad Institute, and will present his findings at international meetings on an annual basis. Formal coursework is planned in grant writing, public speaking, and computational biology to further enhance his probability of success as an independent physician/scientist. Dr. Wein's immediate career goals include acquiring the skills described in this grant proposal in conjunction with his mentor Dr. Kronenberg and collaborators identified within, and publishing first authors manuscripts in order to gain name recognition and to establish himself in the osteocyte field. Dr. Wein's long-term career goal is to establish himself as an independent investigator studying basic cell biology mechanisms controlling bone cell function in vivo. His PhD work with Dr. Laurie Glimcher focused on osteoblast biology, but at this point he needs additional training in skeletal biology and studying osteocytes in vivo and in vitro. Substantial expertise in these areas is present within MGH Endocrine Unit, a collaborative group dedicated to mineral ion metabolism and bone biology, with close attention to relevance to the understanding and treatment of human disease. Aims 1 and 2 of this grant proposal will allow Dr. Wein to gain experience and expertise in the in vivo approaches required to study osteocyte biology. Aim 3 will facilitate the discovery of new genes important for osteocyte function: as an independent investigator, Dr. Wein will then use the skills acquired in this grant proposal to determine the phenotype of novel mutant strains.